There are a couple of things at play. Firstly, eBay is encouraging Stores, and it's far cheaper to list for 30 days in a Store than 7 days as an Auction.
So, sellers can put up a high price and just wait it out - costs almost nothing (especially for media like books at 5c per month).
The "urgency" of the auction format is fading, and buyers are not bidding as much either - they want it Now Now Now!
That said, there is certainly a downward trend in prices of many categories. Technology stuff particularly - the cheapness of imports has really caused local sellers to have to smarten up or miss out completely.
Overall, I reckon the PROFIT in selling on Ebay has dropped considerably in recent years. Or put another way, I am working a lot harder to make the same profits, because I have to turn over about twice as much!
Therefore, like most sellers I am having to reassess the product ranges and find different items to sell. The only bargains from me are the stuff that was overpriced, where I am now marking it down by 25% per month until somebody eventually buys it.